Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades are the primary elements for converting wind energy into electrical energy. The blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between its sides. Consequently, a lift force, which is directed from the pressure side towards the suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During initial start-up of a wind turbine, oscillations occur in various wind turbine components as the generator speed is increased to rated speed. The oscillations of the individual components have a tendency to excite the wind turbine when the frequency of the oscillations equals one of the resonance frequencies of the wind turbine, which is the frequency at which the response amplitude is a relative maximum. As used herein, the term “resonance” is meant to encompass the tendency of a wind turbine component to oscillate with greater amplitude at some frequencies than at others and/or a vibration of large amplitude produced by a relatively small vibration near the same frequency of vibration as the natural frequency of the resonating system or band of frequencies. In addition, the resonance may be due to the coupling effect of the tower with the pitch drive mechanism and/or the coupling effect of the tower with the speed regulator. At such frequencies, even small periodic excitation actions can produce large amplitude oscillations, because the component is capable of storing vibrational energy.
As such, various control technologies have been implemented to control the generator speed of the wind turbine during start-up to avoid components from oscillating at one of their resonance frequencies. For example, various control technologies determine a speed exclusion zone for the generator and prevent the generator speed from operating in this zone for longer than a predetermined time period to avoid exciting the system. The speed exclusion zone of a wind turbine typically refers to a region within the variable-speed region of the wind turbine where the generator is not allowed to operation for sustained periods. Such control strategies, however, are typically only concerned with start-up conditions of the wind turbine and do not consider oscillations caused by high turbulence intensity and/or other environmental conditions combined with wind turbine operational status that occur during subsequent operation.
For example, as wind speeds vary and create turbulence on the wind turbine, the generator speed correspondingly varies and can excite resonance frequencies of various wind turbine components, thereby causing oscillation and/or resonance loads that can damage the wind turbine. More specifically, the rotor blades tend to experience edgewise oscillations or vibrations and/or resonance behavior at high turbulence that increases the blade-edge loads above design loads. Rotor blade edge-wise oscillations occur in the chord-wise direction of the rotor blade between the leading edge and the trailing edge and can damage the blade due to little damping directed to such oscillations.
Further control strategies reduce and/or prevent various wind turbine component loading by shutting down the wind turbine above a certain (cut out) wind speed in an effort to minimize loads. Though this strategy prevents damaging loads that might occur due to the higher turbulence in the wind, a disadvantage is the lack of energy capture in the region above the cut out wind speed. Also, a brief increase in wind speed might trigger a turbine shutdown, while the recovery to normal power production may take some time. On the same token, the occurrence of high turbulence at rated wind speeds will also increase the likelihood of triggering a turbine shutdown. Still further control technologies reduce and/or prevent various wind turbine component loading by measuring a wind speed via a sensor and implementing a control a control action when wind speeds indicate turbulent conditions. Such strategies, however, do not consider resonance and/or oscillation loads as described herein.
Accordingly, an improved system and method for reducing oscillation loads of a wind turbine due to high turbulence and/or combined with other environmental conditions would be desired in the art.